1. Field of the Invention
This invention relates, in general, to electrical inductive apparatus and, more specifically, to vaporization cooled electrical inductive apparatus.
2. Description of the Prior Art
Vaporization cooled systems have been proposed for electrical inductive apparatus, such as transformers, reactors and the like, utilizing two-phase dielectric fluids which have a boiling point within a normal operating temperature range of the electrical inductive apparatus. Dielectric fluid is applied to the electrical inductive apparatus in its liquid state, whereon it evaporates as it contacts the heat producing members and removes heat in quantities equal to the latent heat of vaporization of the dielectric fluid. The resulting vapors are then condensed and reapplied to the heat producing elements in a continuous cycle.
The pressure within the tank of a vaporization cooled electrical inductive apparatus is determined by the partial pressure of the non-condensable gas, such as SF.sub.6, which is used to provide electrical insulation upon startup and also the partial pressure of vaporized dielectric fluid. The partial pressure of the dielectric fluid in its liquid state is small in comparison to its pressure contribution as it boils or is vaporized from the electrical inductive apparatus. Thus, as load is applied to the electrical inductive apparatus and its temperature accordingly increases, a greater proportion of the dielectric fluid will vaporize, thereby increasing the pressure within the tank of the electrical inductive apparatus. An uncontrolled rise in the pressure resulting from a fault could cause a pressure created explosion or rupture of the tank. This poses a problem since the most economical choices for the dielectric fluids in a vaporization cooled electrical inductive apparatus have a toxicity level such that they can be applied only under controlled conditions; but would cause an undesirable or hazardous level of vapors to both humans and the environment in the vicinity of the transformer following a pressure created explosion or rupture of the tank. Thus, it is essential that overpressure rupture of the tank of a vaporization cooled electrical inductive apparatus be prevented.
Typical prior art methods of providing overpressure protection for electrical inductive apparatus, such as transformers, include relief valves and sudden pressure relays. These devices either vent to the surrounding air to reduce the tank pressure or provide electrical contacts for de-energizing the transformer or sounding an alarm. Although satisfactory in conventional oil-filled transformers, the need to contain the vapors within the tank makes the venting type relief devices unsuitable. Furthermore, the sudden pressure relays, being designed to react to a pressure wave in a fluid, are less effective in a gas environment.
Thus, it would be desirable to provide improved overpressure protection for a vaporization cooled electrical inductive apparatus. It would also be desirable to provide improved overpressure protection which prevents the possibility of a tank rupture thereby permitting the usage of economical but toxic dielectric fluids.